1. Field of the Invention
The present invention concerns material as solid electrolytes that can be used for fuel cells using hydrogen ions, that is, protons as charge carriers for the production of hydrogen by steam electrolysis, or sensor material for combustible gases such as hydrogen, as well as methods of manufacturing such material.
2. Description of the Prior Art
Proton conductive solid material is expected as an electrolyte used for fuel cells or for the production of hydrogen by steam hydrolysis. Important properties required for the proton conductive material are that it has high conductivity but does not have any significant contribution of charge carriers (oxygen ions, electrons, etc.) other than protons. Further, since the material is used as a thin film for reducing the electrical resistance, it is also important that the material has no gas permeability and sufficient mechanical strength. In addition, it is also desired that the material is relatively inexpensive from an economical point of view.
Among the proton conductive materials reported so far, hydrates of phosphorus tungstic acid (PWA: H.sub.3 PW.sub.12 O.sub.40.29H.sub.2 O), etc. show relatively high electroconductivity of 4.times.10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 at room temperature. However, such hydrates are not practical for use in fuel cells, etc. though they can be used for sensors, since they suffer from the effects of steam partial pressure and temperature of the atmosphere surrounding them (refer to "Collective Technology for Ceramics Material" by the Edition Committee of Collective Technology for Ceramics Material, p 329-330, published from Industrial Technology Center, on April 10, 1979).
Known proton conductive materials other than hydrates showing satisfactory electroconductivity are those sintered products prepared by partially substituting Ce in BaCeO.sub.3 with Nd, La, etc. or those sintered products prepared by partially substituting Ce in SrCeO.sub.3 with In, Yb, etc. Most preferred proton conductivity has been reported for BaCe.sub.0.9 Nd.sub.0.1 O.sub.3-a in which 10% of Ce is substituted with Nd and oxygen defects are formed to some extent. However, the conductivity in a hydrogen gas stream is only about 1.4.times.10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 at 800.degree. C. and about 7.times.10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 at 600.degree. C. ("Proton Conductive Ceramics and Their Application to Fuel cells" by Hironobu Iwahara, Hiroyuki Uchida, in 1985's Research Report for Special Research under the Scientific Research Funds of Ministry of Education, February 1986).